Biaxially stretched films comprised of polyesters in the stated thickness range are well known.
Electrical applications, such as cables, motor insulation, or films for reverse-side laminates of solar modules, generally require relatively long lifetimes of a number of years, with some exposure to application temperatures which are in the region of the glass transition temperature of polyester (e.g. polyethylene terephthalate): about 78° C. Under these conditions, the susceptibility of the polyesters toward hydrolysis is a critical variable for lifetime in the application. It was shown quite some time ago (McMahon et al., Journal of Chemical Engineering Data, vol. 4 (1), pages 57 to 78, January 1959) that the viscosity here (a measure of the chain length of the polymer) must be at or above a certain level (from 0.3 to 0.33 being critical for IV). The favorable effect of low carboxy end group content (CEG content) on hydrolysis rate has likewise been known for a very long time (e.g. U.S. Pat. No. 3,051,212 of 1962). The processes used in industry for the production of polyesters having low carboxy end group content involve extremely precise process control and subsequent solid-phase polymerization. Polymers of this type are in particular used in the production of high-performance fibers.
The disadvantage of polymers of this type for application in polyester films is that in films it is an economic necessity to use regrind. For reasons associated with the process, the production of biaxially stretched polyester films generally requires from 1.5 to 2.5 kg of polymer for one kg of film. The excess produces edge strips and cut film, which is chopped and then directly reused, or first extruded and repelletized, and then reused. Carboxy end group content rises during film production and then particularly during downstream re-extrusion to produce regrind (repelletized material), and with this there is also a sharp rise in susceptibility to hydrolysis, and this restricts reuse of regrind, and can make this reuse impossible.
Another problem is the difficulty of achieving carboxy end group contents below 15 meq/kg by the post-condensation route. However, these contents are desirable for achieving the markedly reduced hydrolysis rates needed in the abovementioned end uses. If the intention was to achieve this by the postcondensation route, very long condensation times are needed (economically disadvantageous), and very high viscosities become desirable: >0.8 or even >0.85 for IV. However, as viscosity increases it becomes more difficult to process polymers of this type on conventional polyester film plants, since the extruders then draw high current levels and the high viscosity moreover leads to very large amounts of shear-generated heat, which then destroy the initially good hydrolysis properties by forming new carboxy end groups during the extrusion process.
Alongside the method described above for the production of polyesters having low carboxy end group content by way of process optimization and the additional process step of postcondensation (solid-phase condensation or solid-phase polymerization), many processes have been described for endcapping by reactive agents. Ethylene carbonate (e.g. DE-A-19526405, whose United States equivalent is U.S. Pat. No. 5,563,209) or else polymeric reactants, such as carbodiimides (US-A-2002/065346) are often mentioned. Disadvantages of these additives are evolution of irritant gaseous byproducts (particularly in the case of carbodiimides) or the deposition of degradation products or of the endcapping agent itself on the die or in the setting frame. Another factor is that once the endcapping agents have been consumed in the reaction they are ineffective, and formation of the carboxy end groups in the polymer recommences, resulting in the abovementioned regrind problem. The action of polymers and of other polyfunctional endcapping agents, such as carbodiimides, or polymers containing glycidyl groups, as described in EP-A-1 499 668 (whose United States equivalent is United States Patent Application Publication No. 2003/0216500 A1), is not restricted to reducing the number of end groups but also involves chain extension or indeed crosslinking, and this can lead to a viscosity rise that is difficult to control during the extrusion process.
Additives of this type also introduce foreign materials into the polyester, and these can impair the initially very good electrical insulation properties of polyester.
Very generally, additives such as those mentioned above increase costs (costs of the substance itself, and the costs of introduction into the polyester) and give rise to difficulties in process control.
U.S. Pat. No. 3,446,766 describes a method for reducing the number of end groups by using a decarboxylation reaction rather than reactive endcapping with an end group that remains within the product. Polymer production and resultant fibers are described, but there is no description of the production of biaxially stretched films and their use in electrical insulation applications.